1. Field of Invention
This invention relates in general to an electrostatographic imaging member, and in particular, to a process for preparing electrostatographic imaging members and to imaging members produced thereby. In particular, the invention provides a process for enhancing the adhesion of an overcoat layer to the top outermost organic layer of an organic electrostatographic imaging member. Since organic electrostatographic imaging members in a flexible belt configuration require an anticurl backing layer to ensure that the imaging member belt is sufficiently flat, the process of the present invention can also provide improved adhesive bond strength between an anticurl backing layer and a substrate support layer.
2. Description of Related Art
Electrostatographic imaging members are well known in the art. Typical electrostatographic imaging members include, for example, (1) photosensitive members (photoreceptors) commonly used in electrophotographic (xerographic) imaging processes and (2) electroreceptors such as ionographic imaging members for electrographic imaging systems. An electrostatographic imaging member can be in a rigid drum configuration or in a flexible belt configuration, that can be either a seamless or a seamed belt. Typical electrophotographic imaging member drums comprise a charge transport layer and a charge generating layer coated over a rigid conducting substrate support drum. However, for flexible electrophotographic imaging member belts, the charge transport layer and charge generating layer are coated on top of a flexible substrate support layer. To ensure that the imaging member belts exhibit sufficient flatness, an anticurl backing layer is coated onto the back side of the flexible substrate support layer to counteract upward curling and ensure imaging member flatness.
A typical flexible electrographic imaging member belt comprises a dielectric imaging layer on one side of the substrate support layer and an anticurl backing layer coated onto the opposite side of the substrate support layer to maintain imaging member flatness.
The top outermost layer, typically the charge transport layer of an electrophotographic imaging member or the dielectric imaging layer of an electrographic imaging member, is constantly subjected to mechanical and chemical actions with machine subsystems during imaging/cleaning processes. In order to mitigate erosion of the top outermost layer during these processes, the outermost layer can be coated with a thin protective overcoat to provide wear resistance and extend the imaging member""s functional life. Although the present invention applies to both electrophotographic and electrographic imaging members, to simplify the following discussion, the discussion hereinafter will focus only on electrophotographic imaging members, particularly, imaging members in the flexible belt configuration.
In electrophotography, also known as Xerography, including electrophotographic imaging or electrostatic imaging processes, the surface of an electrophotographic imaging member (or photoreceptor) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged. The imaging member is then exposed to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image. This electrostatic latent image can then be developed to form a visible image by depositing oppositely charged particles on the surface of the photoconductive insulating layer. The resulting visible image can then be transferred from the imaging member directly or indirectly (such as by a transfer or other member) to a print substrate, such as a transparency or paper. The image process can be repeated many times with reusable imaging members.
Flexible electrophotographic imaging members can be provided in a number of forms. For example, the imaging member can be a homogeneous layer of a single material, such as vitreous selenium, or it can be a composite layer containing a photoconductive layer and another material. In addition, the imaging member can be layered. Current layered organic imaging members generally have at least a flexible substrate support layer and two active layers. These active layers generally include (1) a charge generating layer containing a light absorbing material, and (2) a charge transport layer containing electron donor molecules. These layers can be in any order, and sometimes can be combined in a single or a mixed layer. The flexible substrate support layer can be formed of a conductive material. Alternatively, a conductive layer can be formed on top of a nonconductive flexible substrate support layer.
In many modem electrophotographic imaging systems the flexible photoreceptor belts are repeatedly cycled to achieve high speed imaging. As a result of this repetitive cycling, the outermost organic layer of the photoreceptor experiences a high degree of frictional contact with other machine subsystem components used to clean and/or prepare the photoreceptor for imaging during each cycle.
When repeatedly subjected to cyclic mechanical interactions against the machine subsystem components, photoreceptor belts can experience severe frictional wear at the outermost organic photoreceptor layer surface that can greatly reduce the useful life of the photoreceptor. For instance, in printers that employ a bias charging roller or a bias transfer roller (BCR or BTR), frictional wear can be so severe that the outer exposed layer""s thickness can be reduced by as much as 10 microns per 100,000 photoreceptor belt revolutions. Ultimately, the resulting wear impairs photoreceptor performance to such a degree that the photoreceptor must be replaced. Replacement of the photoreceptor requires product downtime and costly maintenance.
Typically, manufacturers attempt to minimize frictional wear of the outermost organic layer by applying a protective overcoating to the outermost layer with various materials including nylon materials, such as a crosslinked Luckamide overcoat, so that the photoreceptor is mechanically robust enough to reach a desired product life goal. Unfortunately, although Luckamide and similar materials can provide sufficient protection against frictional wear, such overcoatings do not adhere well enough to the outermost layer of organic photoreceptors to sufficiently extend functional life to avoid the onset of premature overcoat delamination. For instance, although nylon overcoatings have been found to increase photoreceptor wear resistance and increase useful life by as much as four times, to achieve these advantages, it is necessary to heat the overcoat materials to an elevated temperature to bring about a cross-linking reaction to impart sufficient hardness and wear resistance. Although elevation of temperature to achieve total material cross-linking is needed to increase overcoat hardness and to enhance wear resistance, unfortunately, this cross-linking process also leads to poor adhesion between the overcoating and the top photoreceptor layer on which the overcoat is applied. As a result, the overcoating tends to prematurely delaminate, thereby negating the intended protective benefits of the overcoating.
Various methods are generally known in the art to improve adhesion between successive layers in a photoreceptor. For example, U.S. Pat. No. 5,919,514 discloses the use of plasma or corona discharge on an insulating member (substrate) of a donor roll, to increase adhesion and to provide a uniform subsequent metal coating. The disclosed process includes the step of applying corona discharge to the surface of the donor roll, prior to coating the donor roll substrate with a photo or thermally sensitive composition comprised of a polymeric material and a conductive metal nucleating agent.
Similarly, various methods such as plasma discharge and corona discharge are known and used for various purposes. For example, U.S. Pat. No. 5,635,327 discloses the use of glow discharge decomposition to apply amorphous silicon containing at least one of hydrogen and a halogen onto a conductive substrate. Likewise, U.S. Pat. No. 5,514,507 discloses using plasma discharge to form a layer having amorphous silicon germanium as a main body containing at least hydrogen, fluorine and a Group III element.
Another problem commonly associated with flexible photoreceptor belts during extended machine belt cycling is separation of the anticurl backing layer. Premature delamination of the anticurl backing layer from the photoreceptor belt substrate support layer, due to poor interfacial adhesion bond strength, can often reduce the belt""s useful life by as much as 50%. Although various attempts to eliminate premature delamination have been successful, for example U.S. Pat. No. 5,013,624, such measures are generally highly complex and require innovative material reformulations.
Despite the above known methods for improving adhesion between photoreceptor layers, there remains a demand for methods directed to improving interfacial adhesion between a protective overcoating layer and the outermost layer of a photoreceptor onto which the overcoating layer is applied. Because of the above problems, there is an urgent need for effective methods of enhancing interfacial adhesion between overcoating materials and freshly coated outermost photoreceptor layers. There also remains a need for imaging members and photoreceptors having improved adhesion between an overcoating layer and an underlying layer, while still providing acceptable wear resistance to the imaging members and photoreceptors.
Furthermore, there is also a need for a simple innovative approach, for enhancing adhesive bond strength between an anticurl backing layer a substrate support layer, that can be easily adapted and implemented in photoreceptor belt manufacturing.
The present invention is directed to a process for preparing an organic electrophotographic imaging member, either in a flexible belt configuration or in a rigid drum configuration, having at least one charge generating layer and a charge transport layer, wherein the imaging member is added with an overcoating layer having increased interfacial adhesion bond strength between at least an outermost layer and the overcoating layer applied to the outermost layer. The process of the present invention comprises exposing the surface of the outermost organic layer of the imaging member to a corona effluent, and then immediately applying an overcoat layer to the treated outermost layer. Since the corona discharge effluent cleans as well as activates the outermost surface, it increases the outermost layer""s surface energy to improve overcoat solution wetting and can thereby enhances chemical bonding to yield an increase in interfacial adhesion bond strength between the applied overcoating and the treated outermost layer.
In particular, the present invention provides a process for preparing an imaging member, comprising:
applying an organic layer to an imaging member substrate;
treating said organic layer with a corona effluent; and
applying an overcoating layer to said organic layer.
The present invention also provides imaging members formed by such a process. Further, when applied to imaging members of the flexible belt configuration, the same corona effluent surface treatment process can be used to improve adhesion between an anticurl backing layer and an imaging member substrate support layer upon which the anticurl layer is applied.
It is, however, necessary to emphasize that the solvent or solvent mix system used to prepare the applied coating solution should not dissolve the imaging member layer over which the coating solution is applied so that effective adhesion enhancement can be achieved.